SPIGOT-2892: Fix some clone implementations and add unit test

By: md_5 <git@md-5.net>

paper-server/src/test/java/org/bukkit/craftbukkit/inventory/ItemMetaCloneTest.java

@@ -0,0 +1,22 @@
+package org.bukkit.craftbukkit.inventory;
+
+import java.lang.reflect.Method;
+import org.bukkit.Material;
+import org.junit.Test;
+
+import static org.hamcrest.Matchers.*;
+import static org.junit.Assert.*;
+
+public class ItemMetaCloneTest {
+
+    @Test
+    public void testClone() throws Throwable {
+        for (Material material : ItemStackTest.COMPOUND_MATERIALS) {
+            Class<?> clazz = CraftItemFactory.instance().getItemMeta(material).getClass();
+
+            Method clone = clazz.getDeclaredMethod("clone");
+            assertNotNull("Class " + clazz + " does not override clone()", clone);
+            assertThat("Class " + clazz + " clone return type does not match", clone.getReturnType(), is(equalTo(clazz)));
+        }
+    }
+}